Recently, each part of electronic devices has become smaller and thinner, and thus, step caused by the thickness of a wiring pattern itself formed on a substrate, often becomes a problem. Therefore, very thin wires are desired. Conventionally, as a technology for producing a fine wiring pattern, a method for patterning a metal thin film, formed by a heating deposition method or a sputtering method, by photolithography has been used. However, the heating deposition method and the sputtering method require a vacuum environment, and the lithography performed thereafter puts a large burden on drainage and liquid waste disposal. Thus, improvement in terms of environment has been desired. Further, the heating deposition method and the sputtering method require a large number of steps, which leads to a very expensive cost. Therefore, when such methods are applied to form a wiring pattern, reduction of the production cost is quite difficult.
Thus, a technology for producing a wiring pattern by printing, using an ink containing a metal or a metal oxide, has been proposed. The wiring technology by printing enables the production of a large amount of products at a low cost and at high speed, and thus, the technology has already been examined for producing a practical electronic device.
In general, gravure printing is used for printing a thin film. However, the gravure printing requires a large scale equipment, and thus, is not suitable for printing a variety kinds in small amounts. Further, the inkjet printing exhibits a low printing speed, and requires metal particles having a particle diameter of several tens nm or less, when an ink containing metal particles is used. Thus, the cost of raw materials is very high.
Therefore, performing thin film printing by screen printing is desired. However, in case of screen printing, a screen plate has a large transmission capacity, and thus, the thickness of the printed pattern is thick to some extent. In addition, in order to maintain the shape of the printed pattern, the ink needs to have a certain viscosity. For the purpose of increasing the viscosity of the ink, if the metal concentration is increased, the film thickness after the solvent is dried, becomes very thick. Instead, if the viscosity is maintained by a binder resin, while the metal concentration is maintained at a low level, there is another drawback that the conductivity cannot be maintained due to the binder resin. Therefore, as described in Patent Document 1, screen printing is performed by an ink containing inorganic acid silver and/or organic acid silver, an organic binder, and a solvent. However, this method requires a very high temperature of 600° C. or more, thus, cannot be applied to a substrate which is weak against heat, or a substrate mounted with parts.